1. Field of the Invention
The present invention generally relates to the art of cathode ray tube (CRT) displays, and more specifically to a heads-up display (HUD) f or an aircraft including a CRT image generator and a digital look-up table memory which corrects for image distortion in the display.
2. Description of the Related Art
HUDs provide a pilot with information relating to the f light status of his or her aircraft superimposed on a direct visual view through the windshield. The pilot is freed from having to constantly shift sight from the windshield to dials in the cockpit in the conventional manner. This capability can provide a crucial advantage during high speed flight maneuvers, and during take-offs and landings in which conditions can deteriorate rapidly.
As illustrated in FIG. 1, a typical HUD 10 includes a holographic mirror combiner 12 which is built into a pilot's helmet, mounted between a pilot's head 14 and an aircraft front windshield 16 as shown or incorporated directly into a windshield. The flight information image is generated by a system electronic unit 18 to form an image on a screen of a CRT 20 which is projected by an optical system 22 onto the combiner 12 at a slant angle.
The combiner 12 is transparent to the pilot's view through the windshield 16 as indicated by arrows 24, but reflects the image from the screen of the CRT 20 such that it is visible by the pilot superimposed on the direct view. The CRT image can be a radar display generated in a raster pattern, or an information display which may be generated in a raster pattern but is more preferably generated in a stroke pattern.
An exemplary HUD information image 26 is illustrated in FIG. 2, and includes symbols designating roll angle 28, cross track 30, aircraft waterline 32, pitch limits 34, horizon 36, course 38, heading 40, glide slope 42, runway outline 44, altitude 46 and airspeed 48.
CRTs for use in HuDs, as well as other avionic displays, must have flat screens. The image for the HUD display 26 is generated by the electronic unit 18 in an orthogonal raster or stroke pattern to form a visual image on the screen of the CRT 20. However, if no corrective means are provided, the image as projected from the CRT 20 onto the combiner 12 and viewed by the pilot will be distorted. This is unacceptable in applications in which the CRT image must be superimposed accurately onto the direct view.
The distortion inherent in the HUD 10 is caused by several different sources. Due to non-linearity in the magnetic or electrostatic deflection of the electron beam in the CRT 20, an image consisting of orthogonal lines as illustrated in FIG. 3 will be subjected to "pincushion" distortion in which the lines curve toward the center as illustrated in FIG. 4.
Another source of distortion is caused by the slant angle at which the CRT image is projected by the optical system 22 onto the combiner 12, as well as aberrations in the optical system 22 itself. This is known as "geometric" distortion. Assuming that the image is projected onto the combiner 12 from above, the image will appear as curved upwardly and increasing in size in the downward direction as illustrated in FIG. 5.
Certain types of distortion in CRT based displays can be corrected as disclosed in U.S. Pat. No. 3,422,306, entitled "DISTORTION CORRECTION CIRCUITRY", issued Jan. 14, 1969 to S. Gray. Uncorrected analog deflection signals are predistorted in an inverse zaanner to the distortion caused by the CRT to produce corrected analog deflection signals which, when applied to the deflection amplifiers of the CRT, cause the CRT image to be free of pincushion and non-linearity distortion.
The circuitry for predistorting the deflection signals includes a complicated arrangement of analog multipliers, squarers, operational amplifiers and discrete components. Analog systems of this type have inherent accuracy and thermal drift problems, and are expensive and difficult to manufacture. The systems are calibrated and adjusted using analog potentiometers, requiring constant skilled maintenance. In addition, it is extremely difficult to provide a built-in test (BIT) capability in an analog system of this type.